With regard to fiber-reinforced plastics (hereinafter referred to as FRP) which are used as materials that require strength such as molded products and structural materials for buildings, thermosetting resins such as epoxy resins, unsaturated polyester resins, and vinyl ester resins (also called epoxy acrylate resins) are used in combination with inorganic or organic reinforcing materials such as glass fiber or carbon fiber.
Conventionally, prepreg sheets employing a thermosetting resin have been produced by impregnating a fibrous substrate such as glass fiber or carbon fiber with a thermosetting resin containing a non-reactive solvent as a diluent, and simultaneously drying to remove the non-reactive solvent and B-staging the thermosetting resin (thickening) by, e.g., heating; or by impregnating a fibrous substrate such as glass fiber or carbon fiber with a solvent-free thermosetting resin and B-staging by, e.g., heating. Among these prepregs, those of photocurable types have been used particularly for the production of large-size prepregs which cannot be heated or are difficult to heat homogeneously, or for the production of prepregs which are used outdoors. The photocurable-type prepregs have thus become of interest in that they have high curing speed and may simply be applied to buildings without use of any heat source.
There is known a method for B-staging a thermosetting resin in order to produce prepregs wherein a starting thermosetting resin is partially cured to attain an appropriate B stage at which the polymerization is terminated. However, a stable B-stage is difficult to maintain during typical production steps. In particular, when a resin containing a compound having a radical-polymerizable unsaturated group such as an unsaturated polyester resin or vinyl ester resin is B-staged by prepolymerizing the radical-polymerizable unsaturated groups, a stable B stage is difficult to maintain.
In this connection, there is known a method for thickening a vinyl ester resin wherein a hydroxyl group or a carboxyl group pendant from a hydroxyl group in the vinyl ester resin is caused to react with a diisocyanate to attain a B stage. However, using the method for providing a stable B stage is difficult, since the B-staging reaction is largely susceptible to a microamounts of water, active groups, etc.
With regard to typical methods for thickening a unsaturated polyester resin, there are methods involving addition of magnesium oxide which are disclosed in JP-A-62-73914, JP-A-62-73915, JP-A-62-73916, and JP-A-62-73917 (The term "JP-A" as used herein means an "unexamined published Japanese patent application.").
However, the above-described methods for thickening that employ a diisocyanate and magnesium oxide involve a problem that a reactive diluent such as styrene monomer present in a resin does not participate in thickening reaction of a vinyl ester resin or unsaturated polyester resin, to thereby result in separation and sagging of the reactive diluent during storage of the prepreg.
JP-A-6-287524 discloses a method in which a gelling agent is added to a raw material of a prepreg to make the material a disintegrable gel. In the method, however, controlling transition of a gel state to a B stage is still difficult. Thus, problems related to B-staging an unsaturated polyester resin or vinyl ester resin remain unsolved.